An ultrasonic diffraction-grating transducer (DGT), which can be fabricated from piezoceramic, piezoplastic, or any piezoactive material, has the special capability of producing a beam at an angle to its face, as has been disclosed in U.S. Pat. No. 5,540,230 “Diffracting Doppler Transducer” ('230 patent, incorporated herein by reference) and has been used to measure blood velocity, e.g. Cannata J M et al, “Development of a Flexible Implantable Flow Sensor for Post-operative Monitoring of Blood Flow,” Journal of Ultrasound in Medicine, 2012 vol 31, pp 1795-1802 (Cannata reference). The DGT's so described produce a uniform width beam of uniform angle, as shown in FIG. 1. Note beams maintain constant width.
The DGT structure has been useful, particularly for Doppler applications, where its special angled-beam characteristic allows it to be placed on the wall of a vessel and its beam to have a component in the direction of flow in that vessel, allowing Doppler measurements. However, its broad uniform-width beam limits its applications. For example, its broad beam structure, when used as a transmitter, cannot produce a high-resolution image because its “spot size” is large. When used as a receiver, it can only detect energy arriving in the narrow range of angles corresponding to its beam angle—so it is not very sensitive to ultrasound scattered from a point scatterer, e.g. a red blood cell, that spreads the energy it scatters in waves propagating over a wide range of angles.
A focusing device offers the advantage of greater sensitivity as a receiver (because a lens system gathers energy over range of angles) and greater intensity creation as a transmitter because of its focusing action. The importance of focusing ultrasound has led to the development of phased-array ultrasound systems that are the most often used clinical ultrasound imaging systems. These systems use individually connected small ultrasound transducers elements: by firing the elements at carefully chosen different times they can form a focused transmit beam, and by adding individually calculated phase shifts to the signal from each transducer form a focused receive beam. Such systems, requiring separate send and receive channels and cables for each element in the array, which can number in the hundreds or thousands, require expensive transducers, cables to the transducers, and complex circuitry.
DGT's were developed to be transducers that could produce angled beams with a single cable and channel connection. The present invention discloses a DGT that retains the capability of producing an angled beam from a single cable and signal channel, but of increased capability because of its focused transmitting and receiving capability. It is therefore an object of the present invention to improve DGT's so that they have the capability of focusing like conventional phased-array transducers while retaining the capability of producing an angled beam from a single cable and signal channel. It is another object of the present invention to use the focused-DGT for measuring velocity of blood flow in a vessel and imaging applications.